In a wireless LAN (Local Area Network), an AP (Access Point) transmits a beacon packet containing time information (hereinafter referred to as a time stamp) from a clock inside the AP. Then, an STA (STAtion) connected to the AP (that is, the STA having the same SSID (Service Set ID) as the SSID of the AP) receives the beacon packet and matches the time information from the clock inside the STA with the time stamp contained in the received beacon packet. As a result, time synchronization is achieved among a plurality of STAs connected to the same AP.
However, according to such a synchronization technique, if a failure occurs in the AP or if the beacon packet from the AP fails to be received due to, for example, a variation in wireless propagation path, maintaining the time synchronization among the plurality of STAs connected to the AP is difficult.
It is assumed that the plurality of STAs use a certain technique to switch their connections to another AP. However, different APs measure different absolute times, and hence, the time stamp utilized for time synchronization before the switching may differ significantly from a time stamp utilized for time synchronization after the switching.
A plurality of APs that transmit beacon packets may be arranged around the plurality of STAs. However, it is not always appropriate to utilize the average value of the time stamps contained in the beacon packets from the plurality of APs, as a target time for time synchronization. This is due to the following two problems.
First, as described above, different APs generally measure different absolute times, and thus, the absolute value of the time stamp may vary among the plurality of APs. That is, the degree of the effect of the time stamp from each AP on the target time is not uniform. Second, links among the plurality of APs and the plurality of STAs are not necessarily stable. It may be difficult for a certain STA to stably receive beacon packets from a certain AP.